Of the three xylene isomers, paraxylene is the most commercially valuable. Due to the similarity of their boiling points, adsorption, using an adsorbent solid which preferentially adsorbs paraxylene over metaxylene and orthoxylene in a simulated moving bed apparatus, is a common method for separating paraxylene from the other xylene isomers. A commercial embodiment of a simulated moving bed adsorption apparatus is used in the well-known Parex™ Process, which is used to separate C8 aromatic isomers and provide a more highly pure paraxylene from a less highly pure mixture. See by way of example U.S. Pat. Nos. 3,201,491; 3,761,533; and 4,029,717. Other embodiments involving a simulated moving bed adsorption apparatus include ELUXYL™, available from Axens, and AROMAX™, available from Toray.
In a Parex™ unit, the locations of liquid input and output are moved by a fluid directing device. This fluid directing device may comprise one or more rotary valves, as well as various control and accessory means, such as inlet lines, outlets lines, and valves associated therewith. The fluid directing device works in conjunction with conduits connected to adsorbent beds. The fluid directing device accomplishes moving the input and output locations through first directing the liquid introduction or withdrawal lines to specific conduits in fluid communication with particular adsorbent beds. After a specified time period, called the step time, the fluid directing device advances one index and redirects the liquid inputs and outputs to the conduit immediately adjacent and downstream of the previously used conduits. Each advancement of the fluid directing device to a new position is generally called a valve step, and the completion of all the valve steps is called a valve cycle. The step time or step interval is uniform for each valve step in a valve cycle, and may be from about 30 seconds to 4 minutes.
Commercial simulated moving bed adsorption apparatuses such as Parex™ or ELUXYL™ typically contain 24 adsorbent beds and 24 conduits individually connected to a bed and providing fluid communication with the fluid directing device. The conduits of the adsorption apparatus may function, over time, as at least two liquid input lines (e.g., a feed input line and a desorbent input line) and two liquid output lines (e.g., an extract withdrawal line and a reformate withdrawal line).
Systems employing a simulated countercurrent flow process are described in U.S. Pat. Nos. 3,201,491; 3,761,533; 4,029,717; and 8,529,757. Such systems generally include one or more distillation towers and attendant pumps and conduits, which may be utilized to purify the liquid withdrawal streams taken from adsorbent beds.
In standard simulated moving bed separation processes, the flow rate of streams into and out of the simulated moving bed are held constant during the step time. However, modulation of flow during the step time has been found to enhance separation in certain instances involving simulated moving bed separation of fructose and glucose or separation of 1,1′-bi-2-naphthol enatiomers. The enhanced separation may result in greater purity of product streams or less desorbent use. This process for modulating flow rates during a step time has been referred to as a PowerFeed process. Examples of PowerFeed processes are described in an article by Kawajiri et al., “Optimization strategies for simulated moving bed and PowerFeed processes,” AIChE J. Vol. 52 (2006) B, pp. 1343-1350, and in an article by Zhang et al., “PowerFeed operation of simulated moving bed units: changing flow-rates during the switching interval,” Journal of Chromatography A. 1006, pp. 87-99, 2003, Elsevier B.V. The use of PowerFeed operation to separate paraxylene from a mixture of C8 aromatics in a simulated moving bed process is described in International Patent Application PCT/US2015/067701, filed Dec. 28, 2015.
There is an ongoing need to further improve the simulated moving bed adsorption process, maximize the purity of product streams and make the process more efficient. It would be desirable to reduce the number of beds in the unit, thereby reducing the number of conduits and connection devices needed to achieve proper flow of fluids into, out and through the beds of the simulated moving bed apparatus.